Replace pg_class.reltriggers with relhastriggers, which is just a boolean hint

Replace pg_class.reltriggers with relhastriggers, which is just a boolean hint
("there might be triggers") rather than an exact count.  This is necessary
catalog infrastructure for the upcoming patch to reduce the strength of
locking needed for trigger addition/removal.  Split out and committed
separately for ease of reviewing/testing.

In passing, also get rid of the unused pg_class columns relukeys, relfkeys,
and relrefs, which haven't been maintained in many years and now have no
chance of ever being maintained (because of wishing to avoid locking).

Simon Riggs